Searching for high magnetization density in bulk Fe: the new metastable Fe$_6$ phase

TL;DR

This paper reports the discovery of a new metastable iron allotrope, Fe6, with the highest known magnetization density among crystalline iron phases, potentially enabling advanced magnetic applications.

Contribution

The study introduces a novel metastable Fe6 phase with record magnetization density, derived from first principles calculations and structural optimization of Fe3C upon carbon removal.

Findings

Fe6 has the highest magnetization density among known iron phases.

Fe6 is metastable from 0 to 50 GPa but more stable than HCP and FCC at low pressures.

Fe6 can transform into FCC phase under compression.

Abstract

We report the discovery of a new allotrope of iron by first principles calculations. This phase has $Pmn2_1$ symmetry, a six-atom unit cell (hence the name Fe$_6$), and the highest magnetization density (M$_s$) among all known crystalline phases of iron. Obtained from the structural optimizations of the Fe$_3$C-cementite crystal upon carbon removal, $Pmn2_1$ Fe$_6$ is shown to result from the stabilization of a ferromagnetic FCC phase, further strained along the Bain path. Although metastable from 0 to 50 GPa, the new phase is more stable, at low pressures, than the other well-known HCP and FCC allotropes and smoothly transforms into the FCC phase under compression. If stabilized to room temperature, e.g., by interstitial impurities, Fe$_{6}$ could become the basis material for high M$_s$ rare-earth-free permanent magnets and high-impact applications such as, light-weight electric engine rotors or high-density recording media. The new phase could also be key to explain the enigmatic high M$_s$ of Fe$_{16}$N$_2$, which is currently attracting an intense research activity.